Scoliosis is a lateral curvature of the spine which may be caused by, in effect, a buckling of the spinal column during an adolescent growth spurt. In addition to lateral curvature, the vertebrae are rotated with respect to each other. Once adulthood is reached, the spinal column stabilizes, unless the angle of the spine exceeds about 45 degrees.
The current treatment of severe scoliosis is by manually deforming the spinal column and attaching a correction device to the spine such as the Cotel-Dubousset system, as disclosed, for example, in U.S. Pat. No. 5,147,360. The problem with this kind of treatment is that a drastic change in spinal structure occurs over a relatively short period of time, and such change can traumatize the spinal cord resulting in paralysis of the patient.
There has been some efforts toward fixing Nitinol rods directly to the spine, and actuating the rods non-invasively, by high energy electromagnetic waves.
Studies in limb lengthening have shown that nerve tissue in the extremities will viscoelastically stretch if stressed gradually. Peripheral nerve cells cord and become larger. Based on these studies, it is postulated that the spinal column itself might also elongate or stretch if stressed in the same gradual fashion. Thus, the invention here relates to a device which can be fixed to bone by bone screws, and which can exert gradually over a period of time, precise linear increments on specific bone segments in order to either reorient these segments both or stretch them apart. The device itself is designed to be actuated and controlled by external commands, thus eliminating the currently employed direct manipulative procedures required to adjust currently available fixation systems.
The linear actuator in accordance with this invention includes a generally cylindrical, tubular housing which includes a first section and a second section (or expander sleeve) telescopically received within the first section. One end of the first housing section is closed by a threaded plug while one end of the second housing section is formed with an integral closed end. The housing rotationally supports a main drive screw which extends through open ends of the first and second sections and which is threadably received at one end within a nut fixed within the second housing section. The other end of the main screw is rotatably journalled in a bearing fixed within the first housing section. As a result, rotation of the screw results in linear or axial movement of the second housing section or expander sleeve out of the first housing section, thereby serving to axially lengthen the device as a whole.
Both the plug at the one end of the housing and the expander sleeve at the other end of the housing include mounting tabs for fixing the device to specific bone segments by bone screws or the like.
A clutch mechanism is mounted on the main drive screw within the housing and is preferably of a one-way, cammed roller type. In other words, rotation of the clutch in one direction causes rollers to grip the screw shaft and rotate the shaft in the same direction. Rotation in the opposite direction, however, results in a release of the clutch from the shaft so that the clutch can freely rotate without any rotational effect on the shaft. In accordance with this invention, a shape memory alloy component is fixed at circumferentially spaced locations (180.degree. apart) between the housing and a clutch retainer ring, fixed about the clutch, and at the same time, a return spring is fixed at circumferentially opposed locations between the housing and the clutch retainer ring, and acting in the opposite direction. With this arrangement, when the shape memory alloy component is heated, it will contract causing the clutch and the main screw shaft to rotate in the first direction which will result in axial or linear movement of the second housing section or expander sleeve relative to the first housing section to thereby axially lengthen the device. When heat is removed, the shape memory alloy component softens, allowing the return spring to pull the clutch back in an opposite direction of rotation, but without effect on the screw shaft. In other words, removal of heat or cooling of the shape memory alloy component returns the clutch to its original position, but the device itself remains in its extended state. Subsequent heating steps at various intervals will serve to incrementally expand the device even further, in accordance with predetermined treatment procedures.
In an alternative arrangement, the shape memory alloy component is in the form of a generally J-shaped rod spring, one end of which is attached to the housing and the other end of which is attached to the clutch retainer ring. Operation is essentially the same as described above.
In the exemplary embodiment, external radio wave commands and high energy field waves may be utilized to actuate the device. The device will decode the radio waves and transform the electric-magnetic high energy waves to thermal energy which, in turn, will heat the shape memory alloy component.
Thus, in accordance with its broader aspects, the present invention relates to an implantable linear actuator device for use in orthopedic correction procedures, the device comprising a housing including a first tubular portion and a second tubular portion, the second tubular portion slidably received in telescoping relationship within the first tubular portion; and a drive mechanism secured within the housing and connected between the first and second tubular portions, the drive mechanism configured and arranged to convert rotational movement to linear movement to thereby extend the second tubular portion in a direction away from the first tubular portion; and externally actuatable shape memory alloy means connected to the drive mechanism for imparting rotational motion to the drive mechanism.
In another aspect, the invention relates to an implantable linear actuator device for use in orthopedic correction procedures, the device comprising a housing including a first tubular portion and a second tubular portion, the second tubular portion slidably received in telescoping relationship within the first tubular portion; and a drive mechanism secured within the housing and connected between the first and second tubular portions, the drive mechanism including a main screw shaft rotatably journalled at one end in the first tubular portion and threadably connected at an opposite end to the second tubular portion; a one-way rotatable clutch engageable with the main screw shaft such that rotation of the clutch in one direction will effect rotation of the main screw shaft in the one direction but rotation of the clutch in an opposite direction will not rotate the main screw shaft; and a shape memory alloy component fixed between the clutch and the first tubular portion such that, upon application of heat, the component causes the clutch and the screw shaft is to rotate in the one direction.
The device in accordance with this invention reduces the risk of paralysis or pain which might otherwise occur in a one-time surgical procedure. It also serves to allow incremental lengthening of shortened long bones and also allows for compression or distraction forces to enhance healing. It is small in size and provides precise linear displacement under variable loading.
Additional objects and advantages will become apparent from the detailed description which follows.